1. Field of the Invention
This invention relates to improvements in a rotor to be used in combination with a wheel speed sensor which forms part of an anti-lock brake system or a traction control system for an automotive vehicle, and more particularly to the improvements in the structure of the rotor to raise a detection precision of the rotational speed of a road wheel of an automotive vehicle.
2. Description of the Prior Art
An anti-lock brake system (ABS) for an automotive vehicle includes a wheel speed detecting device for detecting rotational speeds of a road wheel. The wheel speed detecting device includes a ring-shaped rotor which is press-fitted around a hub at the side of an axle shaft on which the road wheel is mounted. A wheel speed sensor is disposed facing the rotor, in which magnetic field is developed around the wheel speed sensor and extends throughout the thickness of the rotor. The rotor is formed with a plurality of holes which are aligned at equal intervals in the peripheral direction thereof, so that the peripheral surface of the rotor is formed uneven to take a generally gear shape,. When the uneven peripheral surface of the rotor traverses the magnetic field from the wheel speed sensor, the magnetic flux density is changed so as to develop an electromotive force at the coil of the wheel speed sensor, thereby causing a change in voltage. This voltage change is output as an wheel speed signal to a ABS control unit for effecting an anti-lock brake to the road wheel.
A variety of shapes of the rotors have been proposed and put into practical use. One of them is formed of a ring-shaped sheet metal strip having a plurality of generally rectangular holes which are formed by being punched out. Another one is formed by cutting gear on the peripheral portion of a ring-shaped member. A further one is disclosed in Japanese Utility Model Provisional Publication No. 6-37766 and has a generally ]-shaped cross-section so as to include inner and outer flange sections each of which is formed with a plurality of holes which are formed by punching out each flange section. A still further one is disclosed in Japanese Patent Publication No. 6-14058 and formed by fitting inner and outer rings to each other in which the outer ring is generally comb-shaped. Of these rotors, one produced by punching out the sheet metal strip to form the generally rectangular holes seems to be the most advantageous form the view points of production cost lowering and weight-lightening.
However, drawbacks have been encountered in such a conventional rotor which is produced by punching out the sheet metal strip to form the generally rectangular holes. That is, it is usual that the width dimension of the rotor is generally equal to or slightly smaller than the width dimension of the sensor head section of the wheel speed sensor since the width dimension of the sensor head is usually restricted in connection with the locational relationship between it and other parts around the wheel speed sensor device. Under such a situation, when each hole of the rotor passes by the sensor head section, flat parts located at the opposite sides of each hole simultaneously pass by the sensor head section. As a result, the difference in magnetic flux density or output voltage in the wheel speed sensor is small between a time when the hole with the flat parts pass by the sensor head section and another time when each axially extending flat section located between the adjacent holes passes by the sensor head section. As a result, precision of detection for rotational speed of the road wheel has been unavoidably suppressed at a relatively low level.